(1) Field of the Invention
The present invention relates to LC filters having electrode layers formed on opposite surfaces of a dielectric substrate.
(2) Description of the Prior Art
LC filters having inductor electrode layers and capacitor electrode layers formed on a dielectric substrate are well known. In manufacturing this type of LC filter, an electrode layer arrangement is designed according to an LC circuit the layers are to constitute. When forming an LC circuit as shown in FIG. 1, for example, the electrode layers are accordingly arranged as shown in FIGS. 2 and 3.
More particularly, a capacitor C1 in FIG. 1 comprises capacitor electrode layers 2 and 3 formed on top and bottom surfaces of a dielectric substrate 1 at directly opposite positions across the substrate 1. Similarly, a capacitor C2 in FIG. 1 comprises capacitor electrode layers 4 and 5 formed at directly opposite positions across the substrate 1. An inductor L1 in FIG. 1 comprises a spiral inductor electrode layer 7 formed on the top surface and a spiral inductor electrode layer 8 formed on the bottom surface, which layers 7 and 8 are interconnected via a through hole 6. Similarly, an inductor L2 in FIG. 1 comprises a spiral inductor electrode layer 10 formed on the top surface and a spiral inductor electrode layer 11 formed on the bottom surface, which layers 10 and 11 are interconnected via a through hole 9. Numbers 12 through 14 indicate lead terminals. Lead terminals 12 and 14 are electrically connected to the capacitor electrode layers 3 and 5, respectively. Lead terminal 13 is electrically connected to a connecting land 16 formed on the bottom surface and electrically connected to a through hole 15.
However, where the electrode layers 2, 3, 4, 5, 7, 8, 10 and 11 are formed simply to provide the circuit construction of FIG. 1, it has been practically difficult to realize filter characteristics as designed. This is because floating capacity due to the relative dielectric constant of dielectric substrate 1 affects the filter characteristics.
Where the dielectric layer 1 is formed of ceramics or the like having a high relative dielectric constant, the capacity may readily be obtained by adjusting areas of the capacitor electrode layers 2-5. With the inductors provided by the inductor electrode layers 7, 8, 10 and 11, the floating capacity is caused by the relative dielectric constant of dielectric substrate 1, and the floating capacity and the inductors per se constitute an LC resonance circuit. This resonance circuit deteriorates attenuation characteristics of the LC filter.
Particularly where the dielectric substrate 1 has reduced dimensions for compactness of the filter, spacing between adjacent electrode layers is reduced thereby increasing the floating capacity. As a result, the filter characteristics deteriorate to a greater degree.
FIG. 4 shows attenuation-frequency characteristics of the LC filter formed with the electrode layers as in FIGS. 2 and 3. It will be seen from FIG. 4 that only minor attenuation occurs in a high frequency range with respect to the passband of this bandpass filter. This filter is thus not fit for practical use.